1. Field of the Invention
The present invention is generally in the field of electrical power systems. More specifically, the present invention is in the field of switch mode power supplies.
2. Background Art
Today, power supplies are critical components of many industrial and household electronics. A power supply is sometimes called a power converter and the process is called power conversion. The Power Sources Manufacturers Association's (PSMA) Handbook of Standardized Terminology for the Power Sources Industry defines a power supply as “a device for the conversion of available power of one set of characteristics to another set of characteristics to meet specified requirements.” Power supplies may change the characteristics of the power source voltage and/or current, and include AC to AC, AC to DC, DC to AC and DC to DC power supplies.
For example, from personal computers to hairdryers and cell phone chargers, many electronic items need to convert the 120 Volts, 60 Hz, AC power found in a home in the United States or 220 Volts, 50 Hz, AC power found in other countries to adapt to appropriate characteristics required by the electronic equipment. As an example, personal computers typically require a 5 Volts DC power to operate their internal electronic components, which is provided by a power supply within personal computer that receives and converts the 120 Volts, AC power to 5 Volts DC power.
A switch mode power supply (SMPS) is a power supply that utilizes switches or transistors in conjunction with low loss components such as capacitors, inductors, and transformers. SMPS is known for its low power dissipation, which equates to high efficiency. SMPS has been used for many years in industrial and aerospace applications. Today, SMPS is used extensively in AC powered electronic devices, such as computers, monitors, television receivers, VCRs, etc.
A variety of converter topologies are used in SMPS to regulate power. Today, there are a variety of basic topologies in use, including flyback converters, boost converters, single transistor forward converters, half bridge forward converters, full bridge ZVT converters, etc. As an example, FIG. 1 illustrates conventional half bridge converter 100 along with example related waveforms 130. As shown, half bridge converter 100 includes switching transistor Q1 102, switching transistor Q2 104, capacitor C1 In 106, capacitor C2 In 108, flux balance capacitor 110, half bridge transformer 112, diode CR1 114, diode CR2 116, recirculating diode 118, capacitor C Out 120, and output filter inductor 122. Voltage signals +VIn 124, −VIn 126, +VOut 128, and −VOut 130 are also indicated. In a first stage, when switching transistor Q1 102 turns on, the voltage is reflected across the output windings, and rectified by diode CR1 114, charging output filter inductor 122. When transistor Q1 102 turns off, the voltage drive across half bridge transformer 112 primary drops to zero, and energy stored in the leakage and magnetizing inductances causes a turn-off overshoot, which is clamped by the body diode of transistor Q2 104. In a second stage, switching transistor Q2 104 turns on, and half bridge transformer 112 is driven in the opposite direction, resetting the flux balance in the half bridge transformer 112 core. The output of half bridge transformer 112 is connected to a half wave rectifier, so the alternating polarity pulse train is rectified into a unidirectional pulse train of twice the frequency. Output capacitor C Out 120 and output filter inductor 122 store energy and integrate the duty cycle so that the output voltage is proportional to the product of the rectified output voltage and duty cycle.
Furthermore, SMPS may use pulse width modulation (PWM) or pulse rate modulation (PRM) to regulate the power. For example, television receivers and computer monitors may use either PWM or PRM, while VCRs typically use PRM supplies. PWM SMPS performs its function by varying the “on” or conduction time of the switches or transistors, such that the frequency of the input signal remains constant while the duty cycle varies. As the width of the pulse is increased, the switching transistor stays on longer, and more energy is applied to the switching transformer, which increases the output voltage. Likewise, as the pulse width is made narrower, the transistor is on for a shorter amount of time, and less energy is applied to the transformer. On the other hand, PRM SMPS varies the rate or frequency at which the switching transistor is turned off and on. As the pulse rate increases, the “on” time of the switch decreases. When the switching transistor is turned on and off at a faster rate, less energy is applied to the transformer.
In view of the present state of the art, there remains a strong need for an SMPS topology that can offer a unique control strategy to enable multi-function switches, provide a direct pulse-by-pulse conversion of pulse-width into amplitude, eliminate the dead time between the PWM pulses, eliminate the need for overly complex circuitry, generate less high frequency ripple at the outputs, and improve power density and efficiency.